A quick and easy way is to use sips:

sips -Z 640 example.png

This resizes the image and is therefore less useful.

Do do proper compression in various formats, e.g. avif, ImageMagick is nice.

Install the required tools

brew install libheif

brew install imagemagick

Run the conversion

magick input.jpg -quality 50 output.avif

Some examples

Convert all images from HEIC to JPG

for file in *.HEIC; do sips -s format jpeg "$file" --out "${file%.HEIC}.jpg"; done

Convert all jpg files to webp (or avif)

for file in *.jpg; do magick "$file" -quality 60 "${file%.jpg}.webp"; done

Automatically adjust the orientation of images

magick mogrify -auto-orient -format webp -quality 60 *.jpg